Image forming devices including copiers, laser printers, facsimile machines, and the like, include a drum having a rigid cylindrical surface that is coated along a defined length of its outer surface with a photoconductive material. The surface of the drum is charged to a uniform electrical potential and then selectively exposed to light in a pattern corresponding to an original image. Those areas of the photoconductive surface exposed to light are discharged thus forming a latent electrostatic image on the photoconductive surface. A developer material, such as toner, having an electrical charge such that the toner is attracted to the photoconductive surface is brought into contact with the member's photoconductive surface. The drum then rotates past an intermediate transfer medium where the toner is transferred onto the medium. A recording sheet, such as a blank sheet of paper, is then brought into contact with the intermediate transfer medium and the toner thereon is transferred to the recording sheet in the form of the latent electrostatic image. The recording sheet is then heated thereby permanently fusing the toner to it. In preparation for the next image forming cycle, the member's photoconductive surface is discharged and residual toner is removed.
The toner is stored in a toner reservoir adjacent to the drum. A doctor blade and developer roller are positioned between the toner reservoir and drum for controlling the amount of toner passed to the drum surface. The nip point created between the doctor blade and the developer roller controls the amount of toner transferred to the drum surface. Additionally, the developer roller and doctor blade are electrically charged to charge the toner as it passes through the nip point to assist in the transfer to the drum.
It is important that the doctor blade make uniform and consistent contact across the entire length of the developer roller. If the doctor blade has inconsistent pressure with the developer roller during the transfer, uneven toner amounts will be transferred to the drum resulting in inconsistent and unacceptable print quality. If too much toner is transferred to the drum, printing errors may occur such as blurred images, poor color, and toner particles deposited on the background areas. Conversely, if not enough toner is transferred to the drum, the images will be too light and difficult to see.
A problem in maintaining consistent contact and pressure is the developer roller profile may be non-uniform requiring that the doctor blade move inward and outward to track the surface of the developer roller. Additionally, it is vital that contact be maintained across the entire length of the doctor blade to ensure even print quality across the width of the image. Therefore, it is important that the doctor blade contact and "float" on the developer roller with a consistent amount of pressure for correct toner transfer and toner charge.
One type of previous design permanently mounts the doctor blade against a housing positioned adjacent to the developer roller surface. This design provides for the doctor blade placement to be constant regardless of the surface of the developer roller. However, this design does not address variations and irregularities in the developer roller surface resulting in greater or less amounts of toner to pass by the doctor blade to the drum surface.
Another design provides for the doctor blade to be positioned within a housing adjacent to the developer roller. The doctor blade is attached to the housing to secure the doctor blade in the proper position and angle relative to the developer roller surface. A biasing member positioned behind the doctor blade pushes the doctor blade against the developer roller. One drawback of this design is the doctor blade rubs against the housing during use resulting in friction between the surfaces. This friction reduces the effectiveness in metering toner because a portion of the force by the biasing member is required to overcome the friction and is not available to hold the doctor blade against the developer roller surface. Additionally, as the doctor blade moves inward and outward relative to the developer roller, the friction acts to dampen the blade movement, which results in inaccurate surface tracking.
Another drawback of previous designs is toner leakage around the doctor blade and housing. The contact surfaces between the doctor blade and the housing provide avenues for the toner to collect and pass through to the drum surface resulting in print defects. To overcome this problem, seals may be applied between the edges of the doctor blade and the housing to eliminate toner leakage. Unfortunately, the seals affect the movement of the blade against the developer roller resulting in yet additional force requirement to push the blade against the roller. Additionally, putty is often applied to these areas to further assist in blocking any openings. However, putty causes additional friction between the doctor blade and housing.
Thus, there remains a need for a doctor blade that floats on the developer roller providing consistent toner amounts to be passed to the drum surface.